1. Field of the Invention
The present invention relates in general to a color television system, and more particularly to a method and apparatus for editing image colors in the color television system which are capable of changing the image colors of a broadcasting signal transmitted from a broadcasting station into image colors required by a viewer so that the viewer can watch a picture of the desired image colors on the screen.
2. Description of the Related Art
With reference to FIG. 1, there is shown a block diagram of a basic construction of a typical color television system. As shown in the drawing, the color television system basically comprises a video processing circuit 1, an audio processing circuit 2, a synchronization, deflection and high voltage circuit 3, a color regenerating circuit 4, a convergence circuit 5 and a cathode ray tube (CRT) or a cathode picture tube (CPT) 6.
The video processing circuit 1 includes a tuner circuit 8, a video intermediate frequency amplifying and video detecting circuit 9, a video amplifying circuit 10 and an automatic gain control (AGC) circuit 11.
The tuner circuit 8 has a circuitry similar to that in a black and white (B/W) television system, but with stricter requirements than those of the B/W television system with respect to frequency characteristic, voltage standing wave ratio (VSWR) and variation of oscillating frequency for the purpose of precise regeneration of color.
The video intermediate frequency amplifying and video detecting circuit 9 includes several trap circuits, which are not present in the B/W television system, for preventing a beat interference of 920 KHz due to a frequency difference between a sound intermediate frequency and a chrominance signal.
The construction and operation of the video amplifying circuit 10 is typically as follows:
The single stage, video amplifying circuit converts impedance of a composite video signal inputted therein and then separates a chrominance signal and a luminance signal from the composite video signal of the converted impedance. The chrominance signal is then applied from the video amplifying circuit to a band pass amplifying circuit. At this time, transmission of the chrominance signal from the band pass amplifying circuit to a color output circuit is delayed more than that of the luminance signal due to the narrow band of the band pass amplifying circuit. As a result, there is a necessity for delaying the transmission of the luminance signal for the purpose of coincidence with the transmission of the chrominance signal. For this reason, in the video amplifying circuit, there is included a delay line for delaying the transmission of the luminance signal about 0.8-1.0 .mu.s. However, the delay line has difficulty in obtaining a gain without impedance matching. Therefore, the video amplifying circuit 10 generally may include a four-stage amplifying circuit with further consideration of signal polarity. Further in this circuit, there is included an automatic resolution control (ARC) circuit or a DC restorer, for the further purpose of attenuation of 3.58 MHz signal.
The AGC circuit 11 has circuitry similar to that in the B/W television system, but it must maintain an output voltage from the video intermediate frequency amplifying and video detecting circuit 9 more uniformly than that of the B/W television system in consideration of DC restoration.
The audio processing circuit 2 has the same circuitry as that in the B/W television system. As above-mentioned, the video intermediate frequency amplifying and video detecting circuit 9 includes several trap circuits, which are not present in the B/W television system, for attenuating a sound carrier to prevent a beat interference of 920 KHz due to a frequency difference between the sound intermediate frequency and the chrominance signal. There is little signal component of 4.5 MHz in the output of the video intermediate frequency amplifying and video detecting circuit 9. For this reason, a separate 4.5 MHz detecting circuit is included to obtain a second sound intermediate frequency.
The synchronization, deflection and high voltage circuit 3 includes a synchronization circuit 13, a vertical deflection circuit 14, a horizontal deflection circuit 15 and a high voltage circuit 16.
The synchronization circuit 13 separates a synchronizing signal from the composite video signal from the video amplifying circuit 10, amplifies the separated synchronizing signal and then transfers the amplified synchronizing signal to the vertical and horizontal deflection circuits 14 and 15. The synchronization circuit 13 has the same circuitry as that in the B/W television system.
In the color television system, convergence of luminous flux can come out well only by applying perfectly uniform magnetic force to the Braun tube, due to the large diameter of the Braun tube. For this reason, the deflection coil becomes large in volume and requires a large deflection power. Also, additional circuits are added to extract signals necessary to the convergence of the luminous flux. As a result, vertical and horizontal output transformers have complex constructions. Herein, the horizontal scanning frequency is 15734.263 Hz and the vertical scanning frequency is 59.93 Hz. As known, the frequency difference between the vertical and horizontal scanning frequencies is small and thus not very important.
On the other hand, focus of the color Braun tube is degraded as variation of voltage. As a result, the high voltage circuit 16 is utilized for stability of high voltage.
The color regenerating circuit 4 includes a band pass amplifying circuit 17, a color synchronizing circuit 18, a demodulating circuit 19, a difference signal amplifying circuit 20 and a color output circuit 21.
The band pass amplifying circuit 17 separates and amplifies the chrominance signal of 3.58 MHz .+-.500 KHz from the composite video signal and outputs the amplified chrominance signal to the demodulating circuit 19. In the band pass amplifying circuit 17, there are added a color killer circuit for stopping the band pass amplification in the B/W broadcasting and an automatic color control (ACC) circuit for controlling an amplification degree.
The broadcasting station transmits a horizontal synchronizing signal with a subcarrier (8-12 Hz) being inserted into a back porch of the horizontal synchronizing signal, which is required to demodulate the chrominance signal. As a result, there is a necessity for producing continuously a burst signal, or a color synchronizing signal, being synchronized with the chrominance signal of 3.58 MHz. The color synchronizing circuit 18 functions to produce continuously such color synchronizing signal.
The demodulating circuit 19 detects the chrominance signal synchronously with the subcarrier and extracts color difference signals E.sub.R -E.sub.Y, E.sub.G -E.sub.Y, E.sub.B -E.sub.Y. As a result, the output frequency of the demodulating circuit 19 is not 3.58 MHz, but 0-500 KHz.
The difference signal amplifying circuit 20 amplifies the output signal from the demodulating circuit 19 by a predetermined amplification degree.
The color output circuit 21 subtracts the luminance signal from the difference signals from the demodulating circuit 19 to produce the three primary colors E.sub.R, E.sub.G and E.sub.B.
In a shadow mask type Braun tube, three electron guns are used each of which is slightly inclined inwardly with respect to the center axis of the Braun tube. With this arrangement, a set of three electron beams simultaneously converges on the surface of a shadow mask and pass through apertures formed at the shadow mask to excite fluorescent materials of red, blue and green colors, thereby causing the fluorescent materials to emit light.
Since the distance from the deflection start point to the center of shadow mask is shorter than the radius of curvature of the shadow mask, however, three electron beams are converged on the place before the shadow mask surface, at the shadow mask portions except the center portion of shadow mask, thereby causing them to hardly pass the same aperture. As a result, the reproduced image on the screen may be unsightly, because the chromatic aberration of the image becomes more severe at the screen portions which become more distant from the center portion of screen. For avoiding this phenomenon and making electron beams pass well through apertures at the entire shadow mask portions, it is required to flow current of parabolic, each occurrences wave type having the frequency identical to horizontal and vertical scanning frequencies, through a convergence coil. The circuit forming the above-mentioned parabolic, each occurrences wave is the convergence circuit 5.
As shown in FIG. 2, the CRT 6 is provided with a heater H, a cathode K, a control grid G1, a screen grid G2, a focus electrode G3, an anode electrode G4, prefocus lens L1, a focus lens L2 and a phosphor screen 9. Thermions are emitted from the cathode K heated by the heater H. The emitted thermions are controlled by the control grid G1 and are then drawn by a voltage of the screen grid G2. The thermions passed through the screen grid G2 are focused and accelerated respectively at the focus electrode G3 and the anode electrode G4. Then, the thermions are radiated from the phosphor screen 9. At this time, if the thermions arrived at the phosphor screen 9 are large in amount, the brightness is more strong. As a result, the brightness can be adjusted as variation in amount of the thermions. The amount of the thermions is determined by a relative voltage among the cathode K, the control grid G1 and the screen grid G2. Namely, (control grid voltage)-(cathode voltage)=(bias voltage). Also, as the screen grid voltage is increased, the anode current is increased, thereby making the screen brighter.
The reference numeral 7 in FIG. 1 designates a portion of the color television system corresponding to the output of the chrominance signal.
With reference to FIG. 3, there is shown a detailed block diagram of the portion 7 of the color television system corresponding to the output of the chrominance signal in FIG. 1. As shown in this drawing, the demodulating circuit 19 detects from the output signal from the band pass amplifying circuit 17 in accordance with the output signal from the color synchronizing circuit 18, a red-luminance signal, referred hereinafter to as R-Y, a green-luminance signal, referred hereinafter to as G-Y, and a blue-luminance signal, referred hereinafter to as B-Y. Then, the demodulating circuit 19 outputs the detected R-Y, G-Y and B-Y signals to the difference signal amplifying circuit 20. The difference signal amplifying circuit 20 amplifies the inputted R-Y, G-Y and B-Y signals by a predetermined amplification degree and then outputs the amplified R-Y, G-Y and B-Y signals respectively to a R signal output circuit 21a, a G signal output circuit 21b and a B signal output circuit 21c, in the color output circuit 21. The color output circuit 21 eliminates the luminance signal Y from the inputted R-Y, G-Y and B-Y signals to obtain pure color signals, R, G and B, and then outputs the color signals R, G and B respectively to the corresponding cathode K in the CRT 6.
However, the conventional color television system has a disadvantage, in that it displays the image colors transmitted from the broadcasting station directly through the CRT 6 without any variation. There has recently been required an apparatus for editing the image colors in the color television system, which is capable of changing the image colors variously so that the color television system may be utilized for instance in juvenile education for educating the children how to make a distinction between colors.